Polyimide obtained by reacting tetracarboxylic acid dianhydride with diamine is hereafter expected for a wide use in areas where high-temperature stability is required, because of its various excellent properties and good thermal stability.
In addition to the excellent high-temperature stability, polyimide has also good mechanical strength, dimensional stability, flame retardance and electrical insulation and is broadly applied for the materials of electric and electronic appliances, aeronautics and space instruments and transport machinery; and also used for the high-temperature adhesive of various high performance materials in these fields.
Conventional polyimide, however, normally has an excellent thermal stability with a poor processability due to its high softening point. On the other hand, the resin developed for improving the processability is inferior in high-temperature stability and solvent resistance. Thus the performance of polyimide has both merits and drawbacks.
Generally on the use of conventional polyimide, for example, in film manufacture, wire coating, cover seat or adhesive, solvent removal and imidizing are performed by previously dissolving the polyamic acid precursor into organic solvents and applying on adherends, followed by treating at high temperatures. Insufficient thermal treatment, however, causes property reduction through the residual polyamic acid in the film. Thus the above stated heating period necessarily becomes longer and polyimide forming process becomes very difficult, because slow rise of temperature by step-wise heating and reduction of film thickness are required for avoiding the generation of pinholes in the film. Besides cyclizing reaction by dehydration accompanied with heating has problems, for example, of causing uneven thickness or voids in the film or coated layer. Furthermore, polyamic acid requires cold-storage because it is unstable and its solution results in a gradual gelation or hydrolysis even at the room temperature.
The polyimide adhesive developed by D. J. Progar et al. for high-temperature bonding (U.S. Pat. No. 4,065,345) is known to be excellent in both high-temperature stability and adhesive strength. Processing steps, however, are necessary on its use, where the polyamic acid solution in bis(2-methoxyethyl) ether is applied on the substrate surface and heated to cause solvent removal and cyclizing by dehydration. In addition, high-boiling solvents require selection of processing conditions on the rate and pattern of temperature rise up in order to avoid generation of the pinholes and voids.
Polyimide is required for eliminating problems resulted from the processing method which applies said organicsolvent solution of polyamic acid and heats the coated substrates to cause solvent removal and dehydrating imidization. Moreover, thermoplastic polyimide which can be used for adhesive is desired as it is.